Livestock confinement facilities generate large amounts of animal waste that can create serious environmental and human health concerns. For example, animal waste constituents such as organic matter, nitrogen, phosphorus, pathogens and metals can degrade water quality, air quality, and adversely impact human health. Organic matter, for example, contains a high amount of biodegradable organics and when discharged to surface waters will compete for, and deplete the limited amount of dissolved oxygen available, causing fish kills and other undesirable impacts. Similarly nutrient loading from nitrogen and phosphorus can lead to eutrophication of surface waters. Thus, in the United States through the Clean Water Act and in other developed countries, animal waste may not be discharged but terminally applied to land as a supplement to commercial fertilizer. These requirements do not exist in many other countries having large numbers of livestock and thus, animal waste adversely impacts environmental quality. For example, there is growing evidence of water pollution caused by the discharge of livestock waste into surface waters in various global watersheds and there is now evidence of these discharges affecting coastal water quality such as in the South China Sea, the Gulf of Thailand, and in the Gulf of Mexico.
Animal wastes also impact air quality, which include odor and greenhouse gas emissions. Wastes also contain viruses, bacteria, protozoa, and helminthes that when transmitted to humans can adversely impact human health in a number of ways some of which are life threatening.
A variety of technical approaches have been used to abate these concerns to varying degrees. At the most basic level, wastes are stored and land applied at agronomic rates to reduce nutrient loading and run-off potential. However manure storage does not stabilize waste and therefore does not reduce odor, pathogens, or oxygen demanding materials. More complex processes may use a combination of pre-treatment, primary, secondary, and tertiary treatment processes to provide comparatively superior levels of managing environmental and human health related concerns. Primary treatment is an essential first step when secondary and tertiary processes are considered as primary treatment reduces oxygen demand, reduces pathogens, converts nitrogen and phosphorus into plant available forms, specifically ammonia N and phosphate. Plant available forms of nutrients ensure uptake with a high level of predictability when applied at agronomic rates relative to crop type.
Typically anaerobic processes are used in primary treatment of livestock and other high strength organics as they are economically desirable when compared to aerobic methods. Anaerobic processes transform manure into a variety of end products, including digester effluent and biogas. Various anaerobic systems have been used commercially depending on livestock type, climate and water usage.
One of the most common anaerobic systems used for the treatment of dilute manure is an anaerobic treatment lagoon. In lagoons or any other unmixed systems, materials stratify into solid and liquid components. Sludge (biologically degraded solids) accumulate at the bottom of the lagoon and is composed of settled non-biodegradable and fixed constituents of manure, and active and dead microbial cells. Sludge is black, moderately viscous, typically about 10 percent solids and 90 percent liquid, and high in nutrients, bacteria, and organic matter. Sludge is the byproduct of biological anaerobic degradation or the biodegradable component of organic material. Sludge can be removed manually or by pumps designed for higher solids applications i.e., 10 to 15 percent solids.
The layer above the sludge is the liquid layer. This liquid, the digester effluent, is low in solids (generally 0.3 to 0.6 percent solids), moderately rich in nutrients and easily pumped with irrigation pumps. If the liquid and sludge are mixed, the solids content will range between <1 percent and 8 percent solids, depending on the proportion of process water, rainfall and sludge in the system. The digester effluent and sludge will contain all of the remaining (that which is not volatilized to air) nitrogen, phosphorus, potassium, micronutrients, and metals in the original manure. These can be further processed or land applied.
Unfortunately, anaerobic treatment lagoons while effective in stabilizing organics, are open systems and can emit odor, volatile organic compounds (VOC's), and a number of other constituents into the air that are of growing concern. These gases consist of methane, a greenhouse gas with a warming potential 23 times that of carbon dioxide; ammonia and VOC's which are prerequisite gases in the formation of fine particulate matter (smog), and hydrogen sulfide, an odor compound, which can also cause death in high concentrations. However, when this biogas consisting of about 70% methane is captured in various types of anaerobic digesters and utilized for its energy value, it can provide financial benefit by offsetting energy costs while reducing the air impacts by various combustion processes that destroy methane and hydrogen sulfide.
This biogas can be burned for heat or used to fuel an electric generator among other uses. The heat and electricity can be used on the farm or sold to others. As used herein, a “continuous biogas system” refers to the continuous feeding of biogas to a biogas combustion device such as flares and engines for operation thereof. A “stored biogas system” refers to the storage of biogas for intermittent combustion and use.
There are a myriad of anaerobic digester systems and scales in use around the world. These include simple unheated systems such as covered lagoons and more complex systems that are heated to about 100° F. or higher. Maintaining higher constant temperature reduces reactor volumes required to treat and stabilize waste. A conventional anaerobic digester system generally includes the following components: manure transfer and mixing pit, a digester made of steel, fiberglass, concrete, earth or other suitable material (including heating and mixing equipment if needed), biogas handling and transmission, and gas end use (combustion) equipment such as electric generation equipment. Conventional anaerobic digesters can also require significant operational oversight depending on operational mode and temperature. Conventional anaerobic digester systems also require proper design and sizing to maintain critical bacterial populations responsible for waste treatment and stabilization for sustained long-term predictable performance. Sizing requirements are based on hydraulic retention time (HRT), and loading rate where the operating temperature affects these sizing parameters. These factors (size, materials, operational requirements) affect digester costs, which may be fairly capital intensive and in some economies and farm scales may not be affordable or may be inoperable if experienced technicians are not available.
The issues of affordability and operational ability are exacerbated in developing countries or countries with economies in transition. These countries are predominantly located between 35° north and south latitude where the range of farm scales may be very small household farms to very large corporate production oriented types of farms. This range may encompass farm scales of 5–100,000 pigs or 1–10,000 milk cows per facility. These regions are also showing signs of severe environmental degradation, particularly water and human health, due to large population growth and concentration. These regions to various extents have promoted various anaerobic digesters operated at ambient temperatures constructed from an array of locally available materials to control costs and more expensive and operationally complex systems for larger scale farms. In many cases these smaller systems use low quality materials of limited durability and lifetime to control cost and the larger scale systems may not be cost effective or transportable. Moreover the sizing methodologies used predominantly for smaller scale systems are based on reactor volumes to meet the daily gas requirements for a household or farm (biogas is about 20 cu. Ft/person/day). This approach typically results in only partially stabilizing the waste stream as bacterial populations are subject to washout, short circuiting, and/or excessive loading rates. Furthermore these systems accumulate solids with no provision for removal. Solids and/or sludge accumulation reduce reactor volume and HRT and increase the loading rate causing decreased gas production, increased CO2 concentration in gas stream and/or system failure. The operating track record of these systems has shown marginal to poor performance. While the need to provide affordable technology in the marketplace is essential, it is also essential to integrate quality with performance that enhances the environmental and sanitary conditions for both human and animal health.
Accordingly, there has been a need for a novel improved anaerobic digester system and method for treating animal waste that are predictable, effective, durable, affordable, simple to operate, portable, labor efficient, environmentally friendly, and substantially reliable year-round in tropical and semi-tropical regions located between 35° north and south latitude where these areas have average ambient temperatures of about 65° F. or higher (when at sea level or slightly higher) for passive heating of the digester. There is a further need for a novel improved anaerobic digester and method for primary waste treatment and biogas production for the small, medium, and large scale farms. There is a still further need for a novel anaerobic digester system and method that may be combined with secondary and tertiary processes which increases its environmental performance relative to air, water and human health quality. There is an additional need for a novel improved anaerobic digester system and method that help control air and water pollution from livestock waste, protect public health and offer an opportunity for the waste to be used as a renewable energy resource. The present fulfills these needs and provides other related advantages.